Slot or aperture-coupled planar patch antenna configurations are known for providing antennas having large frequency bandwidth. FIGS. 1A and 1B are an exploded perspective view and a cross-sectional elevation view of an exemplary slot-coupled patch antenna 100 of the prior art. The antenna comprises five major components, namely, a microstrip transmission feed line 103, a ground plane 105, a slot 104 in the ground plane, one or more radiating patches 107, 109 and a metallic cavity 117 (shown only in FIG. 1B). With reference to the Figures, a first substrate 101 has a transmission line 103 formed on one surface thereof and a ground plane 105 formed on the opposing surface. The substrate 101 may be any suitable dielectric substrate on which copper can be deposited or otherwise formed. Substrates typically used for printed circuit board (PCB) applications are suitable. The substrate 101 may be oriented so that the slot 104 is above the transmission line 103 or below it. Either configuration is acceptable as long as the transmission line and the slot are on opposing sides of the dielectric layer 101.
Disposed above the substrate 101 bearing the microstrip transmission line and slot is one or more patch antennas 107, 109. The patch antennas are disposed in additional substrates 111, 113. The patches also are copper layers deposited or otherwise formed on the surfaces of the substrates 111, 113. The substrates provide vertical spacing between each of the patches 107, 109 and between the patches and the slot 104 and transmission line 103. The terms vertical and horizontal as used herein are merely relative to each other and are not intended to connote absolute directions. As shown in FIG. 1A, the dielectric layers 111, 113 may comprise a plurality of layers 111a, 111b, . . . , 111n and 113a, . . . , 113n of conventionally available materials and thicknesses in order to provide the desired vertical distances between the patches, slot, and/or microstrip. The optimum vertical spacings between the microstrip feed line, slot, and patches depends on the desired operating characteristics of the antenna, including, for instance, center frequency, and/or bandwidth. Typically, another dielectric layer 115 will be placed above the topmost patch in order to safely enclose all of the operational components of the antenna (the layer or simply radome). In addition, below the layer 101 bearing the slot and the microstrip there must be a metallic cavity 117 having a depth Dc equal to one-quarter wavelength of the center frequency of the antenna. The metallic cavity is shown in cross section in FIG. 1B but is omitted from FIG. 1A. In operation, energy is fed into the antenna 100 via microstrip transmission line 103. The energy electromagnetically couples from the microstrip 103 to the slot 104 on the opposite side of the substrate 101 and, therefrom, to the patches 107, 109.
The slot 104 radiates in both directions, i.e., up and down. The radiation headed in the down direction, i.e., away from the slots, would be lost in the absence of the metallic cavity 117. Furthermore, it likely would couple to and interfere with the operation of other antennas or circuits in the vicinity. Particularly, these types of planar antennas typically are employed in arrays of multiple antennas in close proximity to each other.
Accordingly, the metallic cavity 117 is provided on the opposite side of the slot 104 from the patches 107, 109 and is about one quarter wavelength in depth. Particularly, the downwardly directed radiation from the slot 104 will be reflected back upwardly by the bottom surface 117a of the metallic cavity. This will prevent the radiation from escaping from the cavity and interfering with other antennas or circuits. Furthermore, the round trip from the slot to the reflecting surface back to the slot, therefore, is one-half wavelength. In addition, the metal reflecting surface at the bottom of the cavity provides another 180 degrees phase shift. Hence the total phase shift is 360° (or 0°) degrees. Accordingly, the reflected radiation will be in phase with the energy radiated from the slot at that moment so that the radiations will superpose with each other increasing the strength of the radiation in the upward direction toward the patches (i.e., the signals add constructively).
While this type of planar antenna has many good qualities, it also suffers from some significant disadvantages. Most notably, the requirement for a one-quarter wavelength metallic cavity causes the antenna to have a significant height. For instance, in a typical application for a planar antenna, such as an automotive application, cellular telephone, satellite radio, or space-based radar one quarter wavelength of typical operating microwave frequency of about 10 GHz would be 7.5 mm. This might render the design unsuitably tall for many applications, including automotive applications, where a low profile is important.
Accordingly, antenna designs have been developed that do not require a quarter wavelength metallic cavity. For instance, Wong, H. et al., Design of Dual-Polarized L-Probe Patch Antenna Arrays With High Isolation, IEEE Transactions on Antennas and Propagation, Vol. 52, No. 1, p. 45-52, January 2004 discloses an L-probe coupled patch antenna that can provide a large frequency bandwidth. FIGS. 2A and 2B are top and cross-sectional side views, respectively, of a dual-polarization L-probe antenna of this design. In a proximity coupled or L-probe antenna 200, there are no slots or metallic cavities. Rather, the end of the microstrip feed lines 201a, 201b are electrically connected by means of vertical vias 203a, 203b through one or more dielectric layers 205 (shown as air in FIG. 2B) to narrow horizontal probes 207a, 207b vertically spaced from the feed line in the direction of the patch(es), e.g., upwardly. The feed energy from the microstrip lines 201a, 20ab travels up the vias 203a, 203b and into the probes 207a, 207b. The probes 207a, 207b direct the feed energy upwardly from the feed line in the direction of the patch 211 to proximity couple to the patch. There is no downward radiation as there are no openings (like slots) in the ground plane of the antenna.
However, while proximity coupled or L-probe coupled antennas can be made thinner, they also have several significant drawbacks. First, they suffer from poor cross polarization. Furthermore, in the case of dual polarization antennas, the isolation between the two polarizations is very poor.